The present invention relates to a method of controlling an electric valve drive device which opens and closes an intake or exhaust poppet valve, and a control system therefor in an internal combustion engine.
A valve drive system in an ordinary engine mainly comprises a cam shaft, a rocker arm (or a tappet), a valve spring and a valve spring retainer, and rotation of the cam shaft driven by a crank shaft is transferred to a poppet valve via the rocker arm to make opening/closing movement.
Output performance and fuel efficiency of an engine generally depend on intake and exhaust efficiency, and the higher they are, the higher engine performance is obtained owing to smooth gas exchange in a cylinder.
However, an automobile engine has broad rotation range, so that it is difficult to improve engine performance over the whole operation range. If high speed performance is increased, low speed performance is decreased, and if low speed performance is increased, high speed performance is decreased.
To solve the problem, there is high-speed valve timing wherein lifts of intake and exhaust valves become larger to increase intake/exhaust efficiency to have larger overlapping area, while there is a small overlapping area valve timing wherein lifts become smaller to form a strong swirl at low speed where combustion is likely to be unstable.
Recently, to meet such requirements, an engine which has a valve operating mechanism of both low and high speed performance, or a variable valve timing lift mechanism which has two valve timing and lifts for low and high speeds is utilized.
However, the variable valve timing lift mechanism is basically similar to the mechanical valve operating mechanism which has the above cam shaft as drive source, and it is thus impossible to remove the performance decrease factor which such valve operating mechanism inherently has, or the performance decrease factor caused by mechanical loss and followability of a valve to a cam.
Valve timing and lift are determined by phase and profile of a cam, and it is impossible to vary them over the whole working range of the engine. There is actually no choice but to set two valve timings and lifts for low and high speed as above.
To solve the problem, as disclosed in Japanese Patent Laid-Open Pub. Nos. 10-37726 and 10-141928, it has been suggested to provide an electromagnetic valve drive device in which a valve is opened and closed by magnetic force instead of mechanical valve drive system having a cam shaft. But, in such a electromagnetic valve drive device, the valve is merely opened and closed by attractive force of an electromagnet, thereby increasing seating noise and providing low responsiveness during operation.
Furthermore, because the control area for valve timing and lift is small, it is difficult to obtain optimum valve timing and lift corresponding to any working condition of an engine, and it would be impossible to improve engine performance over the whole working range.
There is a moving iron core or piece in the valve, so that inertial mass is increased during opening and closing of the valve to decrease responsiveness and reliability at control.
To overcome the disadvantages in the foregoing electromagnetic valve drive device, the applicant suggested an electric valve drive device in which a valve is driven by an electromagnetic actuator called a voice coil motor.
A moving coil is wound around a valve stem, and inside or outside the moving coil, a magnet is fixed so that magnetic flux may be generated in a direction perpendicular to winding of the moving coil. When an electric current is applied to the moving coil, the valve is driven by axial force according to Fleming's left hand rule.
The electric valve drive device provides advantageous large control area of valve timing and lift, but there is problem how to control valve timing and lift in an optimum condition corresponding to any working condition of an engine.
Furthermore, to control the electric valve drive device depending on the working condition of the engine, it is inevitable to provide a valve position detector for detecting actual position of the valve. There are difficulties in mounting position, detecting accuracy and durability against heat.